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Evolutionary Theory
A Theory to Explain Change Over Time
Modern Evolutionary Theory
• A theory is: a broad range of data that has been
scientifically tested and supported.
• Modern evolutionary theory began when Charles
Darwin presented evidence that evolution happens
and offered an explanation of how it happens.
Evolutionary Theory Keeps Changing
& Expanding
• Like most scientific
theories, evolutionary
theory keeps developing &
expanding.
• Many scientists since
Darwin have tested &
added to his ideas.
Peter &
Rosemary
Grantresearch
Galapagos
Finches
Darwin’s Ideas From Experience
• In Darwin’s time, most people did not think that
living things had changed over time.
• In fact, many doubted that Earth itself had ever
changed.
• But Darwin saw evidence of gradual change.
• Darwin’s experiences provided him with
evidence of evolution at work.
The Voyage of the Beagle
Galapagos Islands
• Darwin collected
several different
species of birds called
finches.
• Each of the finches are
very similar, but have
different bills.
• Each bill is suited to a particular food
available on the individual islands.
Galapagos Plants and Animals
• Darwin noticed that
many of the islands’
plants and animals
were similar but not
identical to species in
South America.
• He suggested they
were descendants of
S. American species
with modifications.
Darwin’s original notes on descent
with modification to explain the
different species of birds, tortoises,
etc. seen on the Galapagos.
Descent with modification = Evolution
Darwin suggested
that all species
have descended
from ancient
species with
modifications to
their environment.
• Each of the Hawaiian honeycreeper species has a
bill specialized for eating certain foods. Scientists
suggest that all 23 honeycreeper species apparently
arose from a single species that migrated to Hawaii.
Years of Reflection
• After returning from his
voyage, Darwin spent
many years studying
his data.
• He did not report his
ideas for many years.
• He took time to gather
more data and to form a
strong explanation for
how evolution happens.
Breeding & Selection
• Darwin bred exotic pigeons
and studied the work of
others who bred dogs,
orchids, crops, and other
livestock.
• Breeders take advantage
of natural variation in traits
within a species.
• This is artificial selection.
Artificial Selection – The human practice of
breeding animals or plants that have certain
traits.
Natural Selection
• Darwin suggested
that nature “selects”
which organisms
survive and
reproduce.
• Survival of the Fittest
Darwin’s Ideas from Others
• In Darwin’s time, most
people—including
scientists—believed that each
species was created once
and stayed the same forever.
• But, this view could not
explain fossils of organisms
that no longer exist, such as
dinosaurs.
Do Organisms Change over Time?
• Some scientists tried to
explain such
observations by saying
that species could die out
but never change.
• Others proposed various
mechanisms to explain
how species may change
over time.
Lamarck
• In 1809, the French scientist,
Jean Baptiste Lamarck
proposed an explanation for
how organisms may change
over generations.
• He noticed that organisms
are usually well adapted to
their environment.
…Lamarck
• Lamarck proposed (like Darwin) that
organisms change over time as they adapt
to changing environments.
• The mechanism for that change, however,
that was proposed by Lamarck was wrong.
• Lamarck proposed that changes due to use
or disuse of a characteristic would be
passed on to the offspring. (Is this
possible?)
• Ex: building muscles would mean their baby
would have bigger muscles.
Lamarck’s Idea
of Acquired
Characteristics
was later
shown to be
incorrect!
Use and Disuse
• Lamarck proposed a
reason why giraffes
have a long neck:
• He said that giraffes
stretched their necks to
reach leaves high in the
trees. Their offspring,
therefore would have
longer necks.
• Is this possible?
The Theory of Evolution by
Natural Selection (See page 381)
Natural Selection
• The process by which individuals that are better
adapted to their environment survive & reproduce
more successfully than less well adapted
individuals do.
4 Steps to Natural Selection
1. Overproduction – Every population is capable
of producing more offspring than can possibly
survive. Therefore, there is competition for
survival.
2. Variation – Variations exist within every
population. Much of this variation is in the form
of inherited traits. Some traits are better adapted
than others.
3. Selection – In a given environment, having a
particular trait can make certain individuals more
likely to survive & have successful offspring than
others.
4. Adaptation – Over time, those traits that
improve survival & reproduction will become
more common in a population.
Why do giraffes have long legs
and necks?
• What would the
Theory of Evolution
by Natural
Selection state
about why giraffes
have longer legs
and necks?
Natural Selection Animation
• http://www.mhhe.com/biosci/esp/2001_gbi
o/folder_structure/ev/m2/s1/evm2s1_6.htm
Definitions to Know!
• Evolution: Change in a species over time;
process by which modern organisms have
descended from ancient organisms
• Descent with modification: principle that
each living species has descended, with
changes, from other species over time
• Theory: well-tested explanation, supported
by substantial evidence, that unifies a
broad range of observations
More Definitions to Know!
• Lamarck’s Idea of Acquired Characteristics:
organisms change over time because they
acquire stronger characteristics for the traits they
use often. There is no basis in heredity that
accounts for these changes—they cannot be
passed down to offspring
• Natural Selection:process by which individuals
that are better suited to their environment
survive and reproduce most successfully; also
called survival of the fittest